问题描述

• 在使用dubbo调用接口的时候，莫名其妙出现java.lang.ClassCastException: java.util.HashMap cannot be cast to xxxx异常
• 经过排查发现，是因为dubbo接口返回的不是xxxx对象，而是HashMap

源码分析

• dubbo的反序列化机制默认是hessian2
• 首先定位到SerializerFactory类的getDeserializer()方法

try {
//  Class cl = Class.forName(type, false, _loader);Class cl = loadSerializedClass(type);deserializer = getDeserializer(cl);
} catch (Exception e) {log.warning("Hessian/Burlap: '" + type + "' is an unknown class in " + _loader + ":\n" + e);_typeNotFoundDeserializerMap.put(type, PRESENT);log.log(Level.FINER, e.toString(), e);_unrecognizedTypeCache.put(type, new AtomicLong(1L));
}

• 断点发现deserializer = getDeserializer(cl);这一行代码抛了异常，所以能看到日志输出

log.warning("Hessian/Burlap: '" + type + "' is an unknown class in " + _loader + ":\n" + e);

• 然后手工试了Class.forName，确实会报java.lang.ClassNotFoundException异常
• 那问题就与类加载器有关了，看看hessian2的类加载器是哪个
• 打开org.apache.dubbo.common.serialize.hessian2.Hessian2SerializerFactory这个类

public class Hessian2SerializerFactory extends SerializerFactory {@Overridepublic ClassLoader getClassLoader() {return Thread.currentThread().getContextClassLoader();}
}

• Hessian2重写了类加载器，为了观察这是哪个类加载器，增加日志输出

public class Hessian2SerializerFactory extends SerializerFactory {@Overridepublic ClassLoader getClassLoader() {ClassLoader classLoaderTest = Thread.currentThread().getContextClassLoader();loadClass(classLoaderTest, "com.xxxx");ClassLoader parent = classLoaderTest.getParent();while(parent!=null) {loadClass(classLoaderTest, "com.xxxx");parent = parent.getParent();}return classLoaderTest;}private void loadClass(ClassLoader classLoader, String className) {try {classLoader.loadClass(className);log.info("load success: {}", classLoader.getClass().getName());} catch (ClassNotFoundException e) {log.info("load fail: {}", classLoader.getClass().getName());}}
}

• 经过测试发现，能正常加载类的类加载器是org.springframework.boot.web.embedded.tomcat.TomcatEmbeddedWebappClassLoader和org.springframework.boot.loader.LaunchedURLClassLoader，两个类加载器都是Spring的
• 而不可以加载到类的类加载器是jdk.internal.loader.ClassLoaders.AppClassLoader
• 所以，到了这里，问题就很明显了，dubbo消费者在反序列化的时候，由于加载不到这个类，所以返回的对象按HashMap处理

解决方案

• 参考org.springframework.util.ClassUtils工具类的getDefaultClassLoader()方法

@Nullable
public static ClassLoader getDefaultClassLoader() {ClassLoader cl = null;try {cl = Thread.currentThread().getContextClassLoader();}catch (Throwable ex) {// Cannot access thread context ClassLoader - falling back...}if (cl == null) {// No thread context class loader -> use class loader of this class.cl = ClassUtils.class.getClassLoader();if (cl == null) {// getClassLoader() returning null indicates the bootstrap ClassLoadertry {cl = ClassLoader.getSystemClassLoader();}catch (Throwable ex) {// Cannot access system ClassLoader - oh well, maybe the caller can live with null...}}}return cl;
}

• 如果Thread.currentThread().getContextClassLoader()返回的是JDK类加载器
• 那么就使用ClassUtils.class.getClassLoader()代替
• 修改后的代码如下

    @Overridepublic ClassLoader getClassLoader() {ClassLoader classLoader = ClassUtils.getDefaultClassLoader();if(Objects.nonNull(classLoader) && classLoader.getClass().getName().contains("spring")) {return classLoader;}ClassLoader classLoaderNew = ClassUtils.class.getClassLoader();log.info("classLoader is wrong: {}, change to: {}", classLoader, classLoaderNew);return classLoaderNew;}

• 可以看到日志输出

[2023-12-20 18:56:48.683] [INFO] [ForkJoinPool.commonPool-worker-84]c.m.v.i.d.h.xxxxHessian2SerializerFactory[56][]- classLoader is wrong: jdk.internal.loader.ClassLoaders$AppClassLoader@73d16e93, change to: org.springframework.boot.loader.LaunchedURLClassLoader@14514713

• 最终dubbo接口可以正常返回Java对象，而不是HashMap了

利用dubbo的SPI机制

• 在resources目录下新建文件META-INF/dubbo/org.apache.dubbo.common.serialize.hessian2.dubbo.Hessian2FactoryInitializer
• 文件内容如下

default=xxxx.xxxxHessian2FactoryInitializer

• 这样可以指定自定义的Hessian2FactoryInitializer
• 在这个Hessian2FactoryInitializer里面创建自定义的Hessian2SerializerFactory

@Slf4j
public class xxxxHessian2FactoryInitializer extends DefaultHessian2FactoryInitializer {@Overridepublic SerializerFactory getSerializerFactory() {Hessian2SerializerFactory hessian2SerializerFactory = new xxxxHessian2SerializerFactory();hessian2SerializerFactory.getClassFactory().allow(RuntimeException.class.getName());hessian2SerializerFactory.setAllowNonSerializable(Boolean.parseBoolean(System.getProperty("dubbo.hessian.allowNonSerializable", "false")));return hessian2SerializerFactory;}
}

• 剩下的就以前面的逻辑全部串起来了

总结

• 虽然找到了问题的原因，也解决了问题，但依然有疑问，如果启动多次，有时候能正常转成Java对象，是一个随机的状态，感觉启动的时候决定了这个hessian2的类加载器
• 遇到一些莫名其妙的问题时，不要慌，耐心的断点，分析代码查找问题的原因
• 解决问题时，最好深刻理解源码的设计，利用原有框架的扩展性，进行问题的修复，这样的代码是最优雅的